Ceramic Technology and Processing, King

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Thin sections

Direct observation

Grain boundary detail

Dislocations

5.0 PHYSICAL PROPERTIES

What has preceded this section leads up to developing the physical properties of the ceramic that are needed to fill an application. This section is concerned with how these measurements are made and discusses eight properties: modulus of rupture (MOR), tensile strength, compressive strength, modulus of elasticity (MOE), hardness, fracture toughness, wear resistance, and thermal shock.

Modulus of Rupture (MOR)

MOR tests are conducted on a test machine where stress is measured as the sample bends until it fractures. Figure 11.66 depicts a lab test machine.

A test machine is essentially a rigid frame with a moving head that imposes stress onto the sample. The machine is very carefully designed and constructed so that movements are smooth and even. Force is measured with a load cell and displacement with an extensometer. Alternatively, deflection can be measured as cross head motion, after subtracting out machine compliance. As seen in the figure, the instrument is computer-controlled. The test machine operates at different speeds that are fairly slow for ceramics: about 0.01"/min. Ceramic grain size influences the size of the test bar. A coarse-grained refractory requires a much larger test bar than a 1 μm grained-dense ceramic. Typically, 1/2"x 1"x 7" test bars broken on a 6" outer span are appropriate for the coarse-grained refractory. For a fine-grained ceramic Mil.Std. 1942 is useful. These test bars are specified in a few sizes, with the 3 x 4 x 45mm bar with chamfered edges most common. This size is tested in four point bending with a 20 mm inner span and a 40 mm outer

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Figure 11.67: Four-Point-Bend Fixture. Used for MOR measurements. (Courtesy of MTS)

As the sample bends, the length between the loading lines changes, becoming shorter on the top and longer on the bottom. Clearance is provided so that the loading pins can roll as these distances change. Fixtures also provide side-to-side flexibility so that the test bar and loading lines comply with one another. These ceramics are brittle materials with a high modulus of elasticity, so they are not forgiving of uneven loading or misalignment. Four-point bending is preferrable to single point as a larger volume of the sample is subjected to the maximum stress. Since fractures in brittle materials are initiated by flaw origins, the chances of having one in the stress zone is higher, and the results are more representative.

Attachments for this type of apparatus are expensive and can quickly impoverish a lab's budget. One attachment that may be needed is the capability of measuring MOR at elevated temperatures. An attachment for this purpose is shown in Figure 11.68.

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Figure 11.68: High Temperature Bend Test Apparatus. (Courtesy MTS)

Data on MOR is almost always analyzed statistically, even though it may be only to calculate the mean and standard deviation. Look at the numbers to check if they are reasonable. When there is an exceptionally good result, either one has made a discovery or, unfortunately, a mistake.

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Tensile Strength

With metals and plastics, tensile strength is normally measured with a dog bone sample, where the thin part of the bone is the gage length. All of the material in the gage length is in the maximum stress zone making dog bones the best way to measure tensile strength. There is a catch when dealing with ceramics. Being brittle and having a high modulus of elasticity, they are very difficult to align and obtain a straight pull. Fancy air bearings and special alignment fixtures have been used, but are not common. Two methods exist for measuring tensile strength that do not use dog bones, however. They are the brittle ring test and the hydrostatic test.

Brittle Ring Test 6

In this test, a disc or ring is placed on the press platen and broken in compression as seen in Figure 11.69.

This test has a couple of nice features. The sample is easy to prepare, and the test setup is very simple, requiring only compression platens. In the reference, results from the brittle ring test are compared with those from both tensile and MOR tests. Comparison is fairly good, but which set of data best reflects the closest fit to the actual tensile strength? Like most tests, when they are used to compare materials of similar type, they are valid within the constraints of the test set.

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Figure 11.69: Brittle Ring Test, Sketch. Can be instrumented with strain gages for calibration.

Hydrostatic

In this test, the sample is a tube that is stressed by forcing a liquid (water) into the tube under pressure until it breaks. Figure 11.70 is a sketch of the experimental setup.

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Figure 11.70: Hydrostatic Tensile Test. Yields true tensile strength measurements.

This is an unambiguous tensile test using a sample shape that is sometimes easy to make. Of course, it is only valid for impervious samples or samples with an impervious elastomer liner. Tubes are made by slip casting, injection molding, isopressing, or by extrusion.

Compressive Strength

Compressive strength is measured on cylindrical (round or square) samples with square and parallel ends. Flat and parallel platens are used to compress the sample. Many ceramics are so strong that they will damage steel platens in the test. To avoid this, place the sample on cemented carbide blocks. Perfect alignment is difficult to achieve, so it helps to use the alignment fixture shown in Figure 6.16. A small piece of paper on both ends also helps to smooth out minute irregularities, as the paper compresses during the test and acts as a cushion. There is a lot of elastic energy stored in strong samples and they explode causing a safety hazard for the operator and others in the room. Shield the sample to prevent this hazard.